The present invention relates generally to ohmic contacts for semiconductor electronic devices. In particular, the present invention is an ohmic contact to p-type II-VI semiconductors.
For many years, wide band gap Group II-VI compound, semiconductors, particularly ZnSe and ZnS, have been identified as promising materials for the fabrication of electroluminescent devices such as laser diodes which operate in the blue/green portion of the spectrum. Because of the wide range of important applications for these devices, considerable amounts of research and development have been devoted to these materials. This research led to the demonstration of several blue/green laser diodes based on II-VI semiconductors. See, e.g., Haase at al., Short Wavelength II-VI Laser Diodes, Conference Proceedings for Gallium Arsenide and Related Compounds, 1991 Institute of Physics Conference Series, No. 120, pp. 9-16, and the Qiu et al. patent application Ser. No. 07/700,580, filed May 15, 1991, U.S. entitled Method For Making An Ohmic Contact For P-Type Group II-VI Compound Semiconductors.
One of the most significant obstacles to the development of blue/green laser diodes has been the difficulty in fabricating low resistance ohmic contacts to p-type II-VI semiconductors. Good ohmic contacts, i.e., those capable of operating at relatively low voltages and having a high current carrying capacity while generating little heat, are necessary for commercially viable II-VI devices. Conventional techniques for fabricating ohmic contacts (e.g., those used for III-V compound semiconductors) utilize a metal such as Au (gold) to produce a small barrier to carrier injection, and/or to dope the semiconductor contact layer with shallow (energy level) impurities as heavily as possible at the surface of the layer. Due to the small barrier height and the high doping level in the semiconductor layer, the potential barriers are so thin that tunneling of carriers through the barriers becomes significant. Most all commercially viable semiconductor devices and integrated circuits employ this approach for current injection.
Unfortunately, it has been determined that these techniques cannot be relied upon to produce suitable ohmic contacts to p-type ZnSe, ZnS, CdZnSe, CdZnS and most other wide band gap II-VI semiconductors of interest. Stable low-barrier metal-semiconductor systems and the ability to achieve very high doping levels are, as of yet, not available for these semiconductors. One exception to these problems is ZnTe, which can be easily doped p-type using As, P, Li or N as dopants. It is also possible to make ohmic contacts for this semiconductor using the conventional techniques described above.
One ohmic contact to p-type II-VI semiconductors and its method of fabrication are disclosed in the Qiu et al. U.S. patent application referred to above. This p-type ZnSe ohmic contact includes a contact layer grown at low temperature within an MBE chamber using a cracked Se (i.e., Se.sub.2) source while at the same time doping the semiconductor material of the contact layer p-type in accordance with the method described in the Park et al. article P-Type ZnSe By Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth, Appl. Phys. Lett. 57, 2127 (1990). In addition to the expected shallow impurities utilized by conventional ohmic contacts, additional electronic energy states are formed in the contact layer. These additional energy states are relatively deep (within the forbidden gap) with respect to the valence band maximum, compared to the depth of the shallow impurity level. These energy states are in effect intermediate energy states located at an energy less than the Au Fermi level and greater than the shallow impurity level. Since the probability of charge carriers tunneling between two given energy states increases exponentially with decreasing distance between the two states, the additional energy states greatly increase the tunneling probability by providing a temporary residence for the carriers and facilitating multi-step or cascade tunneling. Maximum current densities of about 800 A/cm.sup.2, have been achieved through the use of this technique.
Nonetheless, there remains a continuing need for improved ohmic contacts to p-type II-VI semiconductors such as ZnSe, ZnSSe, CdZnSe, CdZnS and ZnS. Low resistance and high current carrying capacity ohmic contacts to semiconductors of these types will facilitate the widespread commercial viability of laser diodes and other devices fabricated from these semiconductors.